The magnetic recording media essentially comprise a support such as a polyester film having provided thereon a coated film (that is, a magnetic recording layer, which is hereinafter referred to as a recording layer) comprising ferromagnetic fine particles (such as .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, their modified iron oxide, or CrO.sub.2) and at least one their modified iron oxide, or CrO.sub.2) and at least one thermoplastic resin such as a vinyl chloride/vinyl acetate copolymer resin, a vinyl chloride/vinylidene chloride copolymer resin, cellulose type resins, acetal resins, urethane resins, or acrylonitrile butadiene copolymer type resins, and on the other surface of the support a backing layer comprising non-magnetic particles and a binder comprising one or more thermoplastic resins. Such a backing layer is provided not only to improve the running properties of the magnetic recording medium (that is, running movement between the medium and a magnetic head for recording and reproduction) but also to provide the medium with antistatic property and, in some special cases, to provide the medium with a light-intercepting property. The objective in providing an antistatic property resides in reducing the attraction of foreign substances to the medium which takes place as a result of a build up of electric charges on the magnetic recording medium, causing drop outs. The objective in providing a light-intercepting property resides in reducing errors in the operation of an optical sensor which detects the end or the position of the recording area of a magnetic recording medium by sensing the light transmittance of the medium.
Conventionally, the binders used in the above described backing layer include, for example, a composition containing a vinyl chloride/vinyl acetate copolymer resin, a polyurethane resin, and a polyisocyanate. The binders having functional groups such as a PO(OM).sub.2 group, a OPO(OM).sub.2 group, a OP(OM).sub.2 group, an SO.sub.3 M group, an OSO.sub.3 M group, a COOH group, a COOM group, an OH group, a CONH group or the like (in which M represents a hydrogen atom, an alkyl group, an alkali metal atom, an ammonium group, and an amion group) introduced into the molecular chains of the above-described resins have been put into practical use in order to homogeneously disperse the non-magnetic particles in the backing layer (as described, e.g., in Japanese Patent Application (OPI) Nos. 59623/86, 172213/86, 133012/86, 133013/86, 172213/86, and 177524/83) (the term "OPI" used herein means a published unexamined Japanese patent application).
However, the dispersibility of non-magnetic particles in the backing layer using the above described resins as the binders is still insufficient. Therefore, there are problems which cause the surface properties of the backing layer to be inferior and which allow the backing layer is ready to be easily damaged and permit the powder to readily come off, thereby resulting in staining the parts of tape running system of a magnetic recording and reproducing device when the medium runs.
Further, there is also a tendency for drop outs to increase upon recording due to dust and contaminants which are formed by a magnetic layer, a backing layer, or a support undergoing damage or wear, and due to the consequent staining of tape running system.
Accordingly, a binder having such better dispersibility of non-magnetic particles has been desired to develop.